Respiratory infections caused by the atypical pathogens Mycoplasma pneumoniae, Chlamydophila pneumoniae and Legionella spp. collectively account for approximately 15% of all reported community acquired pneumonia (CAP) cases. M. pneumoniae accounts for about 15-20% of all atypical CAPs. Outbreaks usually occur within a 3-7 year interval with varying incidence rates. Though symptoms are mild and individuals often do not seek medical treatment, severe extra-pulmonary disease can develop if this agent is ignored or misdiagnosed. C. pneumoniae is an obligate intracellular bacterium that accounts for up to 10% of all atypical CAPs. Frequently misdiagnosed or undetected, C. pneumoniae is suspected to be of much higher incidence due to presence of antibodies in 50% of adults due to previous infection. Legionellae account for about 2-8% of atypical community-acquired pneumonia cases. Legionellosis is caused predominantly (in about 70% of all cases) by Legionella pneumophila Sg 1, although other serogroups (Sg 2, 4, and 6) and other species (such as L. bozemanii, L. longbeachae and L. micdadei) have been reported as disease causing agents.
Although a presumptive clinical diagnosis can often be made through symptomology, a laboratory identification determining the etiology is critical to establish the correct course of treatment. These agents are fastidious organisms that require time-consuming procedures, specialized media, and technical expertise for successful culture. Currently, several commercially available serological methods are used to identify these agents. While commonly used, these tests are neither highly sensitive nor specific, and require an acute and convalescent patient serum (paired serum) for clear identification. Frequently these tests fail to identify all acute atypical respiratory infections. Detection by culture is not an effective strategy for diagnosis, as these agents require specific media and expertise, and thus culture identification can take weeks, only allowing a retrospective diagnosis. Urinary antigen tests used to diagnose Legionnaire's disease are only specific to L. pneumophila Sg1, leaving out over 30% of infections caused by other Legionella spp.